The invention relates to an illuminating apparatus for projection purposes, in particular for a film projector or television image projector by the eidophor process, having an all-around radiating source of light, around which, in a plane intersecting its center of radiation, a plurality of condensers aligned with their optical axes on the light source is located, together with deflecting mirrors associated with the condensers and tilted around an axis extending transversely to the path of light in the plane. In such an apparatus, several images of the source of light may be produced on a secondary light source surface located spaced apart from the plane through the source of light, the apparatus further comprising a plurality of adjacent, imaging optical components, with an optical device being coordinated with the secondary light source, whereby the light of the latter is directed onto the image field and the entrance pupil of a projection optic following in the beam path, the images of the condensers being reproduced in the plane of the image field over each other and rotated relative to each other.
An illuminating apparatus of this type is known from DE-A1 1 547 414 and is equipped with a high light intensity source of light by a xenon gas discharge lamp, which is brightest in the vicinity of its cathode. Around the essentially rod shaped xenon gas discharge light four condensers are located in a square when viewed from the top. Each of the condensers consists of a spherical lens and another lens.
The light exiting from the condensers in four directions at right angles to each other is deflected in each case by a deflecting mirror inward in the direction of the axis of the light source and reaches four field lenses contacting each other in a roof like manner, in which the condenser lenses produce images of the light source. The four images of the inhomogeneously radiating light source form in the field lenses a secondary source of light with a star-shaped luminosity distribution, wherein the brightest parts of the images are as close to the optical axis as possible. The optical axis is located in the vertex of the boundaries between the four field lenses in contact with each other. The known apparatus is therefore adjusted so that the brightest areas of the arc images are located in the vicinity of the fields lens borders, triangular in a top view. As the exact location of the brightest areas is critical, slight local displacements of the arc lead to interfering luminosity fluctuations in the subsequent optical devices.
The field lenses reproduce the condenser lenses in an image aperture provided with an image aperture lens, which produces a pattern of the star-shaped secondary light source in the projection optic. In this manner, an effort is made to have the outer circumference of the star-shaped pattern of the light source images extensively approximate the circular configuration of the entrance pupil of the projection optics, in order to attain good utilization of the pupil. If, however, instead of a square image field and a round pupil an elliptical shape of a pupil is present, which is the case for example when a bar system of a schlieren optical device for an eidophor projector distorts the circular pupil of the projection optic into an ellipse, an especially detrimental loss of intensity is experienced in the known illuminating apparatus.